A progressive group is launching an attack ad to brand a candidate in California a "bad Democrat" because he isn't running on "Medicare for all" — another sign of the growing division between center-left Democrats and the party's left wing over single-payer health care.

The digital ads from the Progressive Change Campaign Committee zero in on Dave Min, who has the backing of the California Democratic Party and the centrist group the New Dem PAC in California's 45th District. The race, which has been a bitterly contested primary among Democrats, is considered a vulnerable seat for Republicans in 2018, but rates as leaning Republican.

Min isn't opposed to single-payer health care: He has said he supports "exploring every possible opportunity to achieve universal health care coverage," said he was behind a California plan to create a statewide single-payer system, and supports a public option. But like many Democrats running in red and purple districts in 2018, he's expressed skepticism about a federal single-payer plan like the Bernie Sanders-backed "Medicare for all" bill.

Hutchinson, Min's campaign manager, told the Intercept that Medicare for All would amount to a "tremendous tax increase" that would play poorly in a district that no Democrat has ever won. (She told BuzzFeed News that the quote was inaccurate, and that she had been referring to potential Republican attacks on the plan.)

PCCC's new ad seizes on Min's stance to say he has "sided with big insurers by refusing to embrace" Medicare for all. His opponent, Katie Porter, who's won endorsements from Sens. Elizabeth Warren and Kamala Harris, has said she wants to replace the Affordable Care Act with a federal single-payer system. (Porter actually did economic research for Warren, who also contributed to Porter's own book.)

“The attack is a flat-out lie,” said Paige Hutchinson, Min’s campaign manager. “Dave has consistently said he believes health care is a human right, and that we should try to move as quickly and practicably to universal health care coverage as quickly as possible. Dave is open to different pathways to getting there and has never once said he opposes 'Medicare for all.'”

The ad, a PCCC representative said, is also the beginning of what the group plans to be an ongoing campaign to tar the centrist "New Democrat" label, embraced by Min and others, as tied to corporate interests.

"This is partially about truth in labeling for voters," said PCCC's Adam Green of the ad. "You can't masquerade as a progressive and pretend to reject corporate interests when you take Big Insurance money through the corporate New Dem PAC and then actively attack 'Medicare for all.' Voters deserve to know the clear contrast between Elizabeth Warren's protégé and a corporate New Dem — one will lead on a progressive agenda while the other will attack and undermine it."

"Medicare for all" has won increasingly broad support among Democratic Party leaders, including many top contenders for the 2020 presidential nomination, and is broadly popular among voters. But some polling suggests the plan is not well-understood among voters and is vulnerable to Republican attacks — a consideration for some Democrats in competitive races like Min's.